A storage system is a processing system adapted to store and retrieve information/data on storage devices, such as disks. The storage system includes a storage operating system that implements a file system to logically organize the information as a hierarchical structure of directories and files on the disks. Each “on-disk” file may be implemented as a set of disk blocks configured to store information, such as text, whereas each directory may be implemented as a specially-formatted file in which information about other files and directories are stored.
The storage operating system generally refers to the computer-executable code operable on a storage system that manages data access and access requests and may implement file system semantics in implementations involving storage systems. In this sense, the Data ONTAP® storage operating system, available from Network Appliance, Inc. of Sunnyvale, Calif., which implements a Write Anywhere File Layout (WAFL®) file system, is an example of such a storage operating system implemented as a microkernel within an overall protocol stack and associated disk storage. The storage operating system can also be implemented as an application program operating over a general-purpose operating system, such as UNIX® or Windows®, or as a general-purpose operating system with configurable functionality, which is configured for storage applications as described herein.
A storage system's disk storage is typically implemented as one or more storage volumes that comprise physical storage disks, defining an overall logical arrangement of storage space. Available storage system implementations can serve a large number of discrete volumes such as 150 or more, for example. A storage volume is “loaded” in the storage system by copying the logical organization of the volume's files, data, and directories, into the storage system's memory. Once a volume has been loaded in memory, the volume may be “mounted” by one or more users, applications, devices, and the like, that are permitted to access its contents and navigate its namespace. As used herein, a volume is said to be “in use” when it is loaded in a storage system's memory and at least one user, application, etc., has mounted the volume and modified its contents.
A storage system may be configured to allow clients to access files stored on a storage system. In this model, the client may comprise an application executing on a computer that “connects” to the storage system over a computer network, such as a point-to-point link, shared local area network (LAN), wide area network (WAN), or virtual private network (VPN) implemented over a public network such as the Internet. Communications between the storage system and its clients are typically embodied as packets sent over the computer network. Each client may request the services of the storage system by issuing file-system protocol messages formatted in accordance with a conventional file-system protocol, such as the Common Internet File System (CIFS), Network File System (NFS) protocol, Fibre Channel Protocol (FCP), Internet Small Computer System Interface (iSCSI), or Storage Area Network (SAN) access.
A storage system may be used to implement a backup and/or restore of its file system, such as a backup and/or restore of one or more files of the file system, a subset of the file system, or the entire file system. A backup usually comprises one or more images of the file system or a portion thereof. Each image captures a state of the file system at a selected point in time. The images provide a convenient format for the storage and/or retrieval of several backups of the file system over time, such as for an archive or chronology of backups. Despite the advantages of image-based backups, a conventional backup consumes significant amounts of time and resources of the storage system. For instance, it requires intensive input/output operations of the storage system and requires significant storage space. However, backing up, archiving, and/or restoring the files of a file system are essential to maintaining a file system and its data.
Traditionally, the images generated by some storage systems are static or read-only from the moment of generation. This has benefits in terms of the fixed integrity of the imaged and/or stored data. Some applications, however, may require and request, from the storage system, dynamic images (i.e., images having read-write capability). Accordingly, there is a need for a method of adapting the static image technology implemented by some storage systems to provide a dynamic image, to an application requiring a dynamic image.